Recently, in response to the demand of miniaturization and lightweight of electronic appliances, thinner and higher density circuit boards are being developed. In particular, for electronic appliances used in the fields of information/communication and information processing, higher performance is strongly demanded. Thus, a need for acquiring areas for mounting high performance parts is growing.
Up to present, to ensure such mounting areas, miniaturization of surface mounting parts, narrowing of terminal pitches, fine pattern of substrates, SMT (Surface Mount Technology) in which parts are mounted on a substrate surface at a high density and Advanced SMT which is an upgraded version thereof have been studied.
Recently, however, passive devices (capacitors, inductors and registers) responsible for electrical control occupies more than half of the mounting area compared to active devices (chip parts) contributing to higher function, and this has prevented progress of miniaturization and higher function.
Thus, development of “substrate with embedded passive devices” in which functions of passive devices are built within a substrate is required. In particular, regarding embedding of capacitors which account for more than half of the passive devices, utilization of an insulating layer as a dielectric of a capacitor has been proposed. By integrating functions of passive devices within a substrate, soldered joints conventionally used for electrical connection between surface mounting parts and wiring boards are no longer present. It is thus expected that reliability is improved and more flexible circuit design becomes possible. In addition, it is expected that since integration makes it possible to form a passive device at an effective position, wire length can be shortened and as a result, parasitic capacitance is reduced and electrical properties are improved. Moreover, because surface mounting is unnecessary, cut down of costs is expected.
The capacitance which is the performance of an embedded capacitor is represented by the following formula, and to improve the capacitance, higher dielectric constant of an insulating layer and thinning of an insulating layer are effective.C=∈·∈r·A/t  (formula 1)
In the formula, C: capacitance (F), ∈: dielectric constant in vacuum (8.85 F/m), ∈r: relative dielectric constant of insulating layer, A: electrode area (m2) and t: thickness of insulating layer (m).
As an insulating layer for a printed wiring board, glass cloth prepregs produced by impregnating glass cloth with resin, drying the same and semi-curing the resin have been conventionally used. For multi-layered printed wiring boards, in addition to such glass prepregs, prepregs without glass cloth, i.e., adhesive films obtained by semi-curing resin capable of forming film, are used. These are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 6-200216 and 6-329998. Further, for multi-layered printed wiring boards, an adhesive film with copper foil in which the above adhesive film is formed on one side of copper film is used. This is disclosed, for example, in Japanese Patent Application Laid-Open No. 6-196862.
Conventionally, glass cloth impregnated with thermosetting resin such as epoxy resin or insulating resin coated on copper foil has been generally used. These materials have a dielectric constant of about 3 to 5. Methods of using such materials as a dielectric of an embedded capacitor have been proposed, and disclosed, for example, in U.S. Pat. No. 5,079,069. The dielectric constant, however, was in the single digits as described above. In addition, to increase the capacitance of a capacitor, the thickness of an insulating layer must be reduced.
To achieve thinning of prepregs composed of glass cloth, the thickness of the glass cloth itself, which is a base material, needs to be reduced. At present, thin glass cloths of about 20 μm are available on market, but they have a problem in production that their strength is decreased due to thinning and so the base material is easily broken upon coating. Further, since the cloth is impregnated with resin and then used as an insulating layer, the resultant becomes thicker than the original base material after impregnation, and thinning has thus reached its limit.
Semi-cured adhesive film, which is a prepreg without glass cloth, is generally composed of an organic insulation material containing a thermosetting component. Although thinning of this prepreg is easier than that of a prepreg composed of glass cloth, improvement of dielectric constant was difficult.
As a technique for increasing dielectric constant, for example, an insulation material in which an inorganic filler having a relatively high dielectric constant is combined with an organic material has been studied. It has also been reported that high dielectric constant is achieved by using spherical inorganic filler having an average particle size of 20 μm or 40 μm. This is disclosed, for example, in Japanese Patent Application Laid-Open No. 53-88198. However, despite the effort of thinning of the composite material in order to improve the capacitance, thinning was difficult because the inorganic filler had a large particle. In addition, use of particles having a sufficiently small particle size causes problems that thixotropic properties of varnish upon forming a composite are increased and thus a uniform insulating layer cannot be obtained. Further, in the case of high volume filling of filler for improvement in dielectric constant, resin is not sufficiently mixed with the filler and voids and defects are generated, and this has resulted in problems that dielectric constant is not increased and such defects have a bad effect on reliability and the like upon use as an insulation material.